The invention relates to a dispenser cathode having a cathode body which comprises at least a refractory metal and a rare earth metal-containing material, and to a method of manufacturing a dispenser cathode, in which method a powder of a refractory metal and a rare earth metal-containing powder, in particular a scandium-containing powder, are mixed with each other and formed into a cathode body, while the cathode body is also provided with a barium-containing component.
In this context, the term "rare earth metal" is not limited to the lanthanides, but also includes, for example, yttrium and scandium.
Such a method is known from European Patent Application no. 298558 laid open to public inspection. In the method described in said document, a refractory metal in the form of tungsten powder and a scandium-containing powder comprising pure scandium or scandium hydride are mixed with each other in a ratio of 95:5 percent by weight, whereafter the powder mixture is compressed and sintered to form a cathode body which consists of mainly porous tungsten in which the scandium has been distributed. The cathode body is further provided with a barium-containing component by impregnating the cathode body at an elevated temperature with molten barium calcium aluminate to incorporate an electron emissive material.
Such a cathode is generally referred to as mixed-matrix scandate cathode and comprises a porous matrix mainly consisting of the refractory metal in which oxidized scandium (scandate) is distributed, while the barium-containing component, which usually has an oxidized form, is present in the pores of the matrix.
The oxidized states of scandium and of barium will hereinafter be referred to as scandium oxide and barium oxide, respectively, without exclusively indicating purely stoichiometric compounds, unless explicitly stated. The oxidized states may comprise, for example hybrid forms of stoichiometric oxides, viz. mixed oxides.
The barium-containing component ensures that a mono-atomic layer comprising barium is formed on the emissive surface of the cathode. The barium oxide is then reduced to barium by the matrix metal. Due to the mono-atomic top layer, the work function of free electrons in the matrix is sufficiently decreased to render electron emission possible. Since the mono-atomic top layer continuously loses barium due to the inevitable evaporation of barium, barium is, however, to be dispensed continuously so as to maintain the layer, which accounts for the name of such a cathode. Barium is dispensed in that, during operation, barium oxide which is reduced or not reduced diffuses from the pores to the emissive surface where it replenishes the mono-atomic layer.
In a mixed matrix scandate cathode the electron work function is further reduced in that not only barium but also scandium is present in the mono-atomic top layer. Such a cathode thus has an extremely high efficiency so that a comparatively strong electron emission can be realised already at relatively low temperatures. For example, with a cathode of the type described in the opening paragraph an electron emission of more than 100 A/cm.sup.2 can be realised at a heating temperature of approximately 1000.degree. C., which corresponds to an efficiency which is more than a factor of 10 higher than that of a dispenser cathode which does not comprise scandate. A cathode of the type described in the opening paragraph is therefore eminently suitable for use in an electron vacuum tube, particularly in a display tube in which a picture is imaged on a display screen by means of an electron beam generated by the cathode, or in a camera tube in which image information is read from a target plate by means of an electron beam generated by the cathode.
A problem which may occur in practice in the manufacture of such a cathode is that it is difficult to mix the starting powders. The scandium-containing material and the refractory metal often tend to demix. In addition, particularly very fine powders, i.e. powders having a very small average grain size appear to have the tendency of sticking together, which contributes to a poor mixability of the powders but also leads to poor handling and difficult processing.